<!DOCTYPE html>
<html lang="zc-CN">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
    <script>
       let a = +prompt('请输入银行卡余额')
       let shisu = +prompt('请输入当月食宿消费金额')
       let live = +prompt('请输入当月生活消费金额')
       let shengyu
       function fn(a,b,c) {
            shengyu = a-b-c
            return shengyu
       }
      let res = fn(a,shisu,live)
      console.log(res);
    </script>
</body>
</html>